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POOutput iterator's value_type
 

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    <p>The STL commonly defines an output iterator like so:</p> <pre><code>template&lt;class Cont&gt; class insert_iterator : public iterator&lt;output_iterator_tag,void,void,void,void&gt; { // ... </code></pre> <p><strong>Why do output iterators define <code>value_type</code> as <code>void</code>?</strong> It would be useful for an algorithm to know what type of value it is supposed to output.</p> <p>For example, a function that translates a URL query <code>"key1=value1&amp;key2=value2&amp;key3=value3"</code> into any container that holds key-value strings elements.</p> <pre><code>template&lt;typename Ch,typename Tr,typename Out&gt; void parse(const std::basic_string&lt;Ch,Tr&gt;&amp; str, Out result) { std::basic_string&lt;Ch,Tr&gt; key, value; // loop over str, parse into p ... *result = typename iterator_traits&lt;Out&gt;::value_type(key, value); } </code></pre> <p>The <a href="http://www.sgi.com/tech/stl/value_type.html" rel="noreferrer">SGI reference page of <code>value_type</code></a> hints this is because it's not possible to dereference an output iterator. But that's not the only use of <code>value_type</code>: I might want to instantiate one in order to assign it to the iterator.</p> <p><strong>What alternative approach is there for constructing a value to output with the output iterator?</strong> Two approaches I considered:</p> <ul> <li>Accept a functor parameter that would return an object of the correct type. I still want to have a version of the algorithm that doesn't take that function object parameter though.</li> <li>Require that the output container holds <code>pair&lt;string,string&gt;</code>, or else a type convertible from that. I wonder if I can do without this requirement, perhaps allow any element that can construct from two <code>std::string</code> s.</li> </ul>
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    1. COWhat I'm trying to do is generalize the algorithm such that it works with containers of any type that can construct from two `string` s, not just `map<string,string>`. Then, the template can instantiate into `*result = pair<string,string>(key,value)` as well as `*result = unicorn_pony(key,value)`.
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    2. COIf I may add a reference: a paper [pdf] about output iterators, also complains about this issue. http://semantics.org/publications/02_02_multiout.pdf
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    3. COThe example given in that paper is exactly why output iterators are not required to define a value type. He arbitrarily decided that `MultiOut::value_type` should be `double`, but in reality *any* type that can be implicitly converted to both `double` and `int` are reasonable values to use for the assignment operator.
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      1. USDennis Zickefoose
 

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